THE AMMONIA DIMER SPECTRUM IN COLD HELIUM CLUSTERS

Infrared molecular beam depletion spectroscopy has been used to measur e the spectrum of ammonia dimers embedded in large helium clusters aft er the excitation of the umbrella mode. The spectral region from 940 t o 1010 cm(-1) was covered by a line-tunable CO2-laser. By measuring th e dependence of the depletion signal on the pressure of the pick-up ce ll, we confirmed that all lines are caused by the dimer. The spectrum exhibits two groups of two lines at 978/983 and 992/997 cm(-1) separat ed by 14 cm(-1) with a line splitting of 5 cm(-1) each. The infrared t ransitions are only marginally shifted compared to the free ammonia di mer. Also the inversion tunneling splittings are equal to those of the free dimer. Differences in the amplitudes are attributed to relaxatio n effects of the different inversion components of the same symmetry a t the low temperature of 0.4 K. The interchange tunneling splitting, h owever, which dominates the line splittings in the free dimer is consi derably quenched in the helium environment. In addition, the differenc e in the excitation of the two non-equivalent monomers is remarkably r educed in support of a more cyclic structure in the helium environment . (C) 1997 American Institute of Physics.